That's what I do too - refrigerate/freeze and only stack cakes whose frosting is solid. I also leave the dowels sticking up from the bottom tier so that when I set my upper tier on it, it sinks to the bottom tier and I don't mess up the frosting on either cake. This is especially helpful for fondant covered cakes that don't get refrigerated. Just make sure you push your dowels all the way to the bottom and then pull up, otherwise you could some that refuse to sink in!